The present invention relates to analytical equipment having a function of detecting a liquid surface, and more particularly to analytical equipment capable of detecting a liquid surface on the basis of a change in electrostatic capacitance at a time a probe is put in contact with the liquid surface.
An example of an automatic analyzer, in which a probe acting as a pipette has a function of detecting a liquid surface, is disclosed in JP-A-57-82769. In this example, a pipetting probe provided with a liquid-surface sensor falls toward a liquid surface in a reagent container, and the volume of a reagent remaining in the reagent container is calculated from the moving distance of the probe necessary for the probe to reach the surface of the reagent. In this example, however, a pair of electrodes (that is, a pipetting probe electrode and a liquid-surface detecting electrode) make a vertical motion, as one body. Accordingly, it is impossible to use the above probe in a case where a liquid container has a covering membrane for preventing the evaporation of a reagent or the like. In view of this fact, a liquid-surface sensor of the electrostatic capacitance type has been used in an automatic analyzer.
It is disclosed in U.S. Pat. No. 3,635,094 that after the surface of a liquid substance has been detected by utilizing electrostatic capacitance, a predetermined quantity of liquid substance is sucked by a probe. In this case, a liquid-surface detecting electrode is separated from the probe. In order to simplify such a structure, the probe itself has been used as a liquid-surface detecting electrode.
The latest example of an automatic analyzer provided with a liquid-surface sensor of the electrostatic capacitance type is disclosed in JP-A-62-194464. In this example, a probe which is formed by uniting a liquid-surface detecting electrode and a pipetting tube in one body, is inserted into a liquid container having a covering membrane, and the volume of a liquid substance remaining in the containers is calculated from the height of the container and the moving distance of the probe which is moved at a constant velocity.
In a case where the electrostatic capacitance between a probe which also serves as a liquid-surface sensor, as an electrode disposed under a liquid container is measured, there arises a problem that a high-frequency signal applied to the probe exerts an adverse effect on the operations of other members. The above-referred JP-A-194464 pays no attention to this problem.
Further, in a case where a liquid surface in a container which has a covering membrane for preventing the evaporation of a liquid substance, is detected by a pipetting probe serving also as a liquid-surface detecting electrode, when the probe is inserted into or withdrawn from the container, the probe is frequently put in contact with the covering membrane. For example, let us consider a case where a reagent container has the covering membrane. When the probe is withdrawn from the container, a liquid reagent on the other surface of the probe adheres to the covering membrane. In a case where the liquid reagent in the container is taken out only once, the liquid reagent adhering to the covering membrane offers no problem. In a case where the liquid reagent in the container is pipetted a plurality of times, however, there is a fear that a small amount of liquid reagent adhering to the covering membrane is erroneously detected as a liquid surface in the container. The above-referred JP-A-62-194464 pays no attention to this problem.